Transcript for 'Knee Defender' May Have Led to Plane's Emergency Landing

Two passengers get into a fight and the flight has to be di vered all thanks to those widely available knee defender that preinterhaves the passenger from reclining their seat. ABC's David Kerley with Reagan national airport's story. Reporter: It's these two little pieces of plastic that caused this heated argument, caused the diverse of a jetliner and that unplanned landing. It was passenger against passenger on a united airlines 737 from Newark to Denver forced to land in Chicago. The problem, reclining seats. Or rather the lack of reclining seats because of a $22 product brought on board. The two passengers in middle seats in economy plus. Those are the seats you pay extra for a bit more legroom. The man in 12b using what's called a knee defender on the seat back in front of him. Those brackets placed on the tray table keep the seat in front from reclimbing. That meant the woman in 11b could not lean back and she was not happy. An argument over personal space until she reportedly took a cup of water throwing it into the face of the man behind her. This has been on the market for 11 years next month, never happened before. Reporter: He should know. He's the one who invented the knee defender. Knee defenders aren't about getting more space. They're about stopping something from moving and hitting you. It starts the conversation before there's a problem. Reporter: Well, that's not the way it worked on the united flight. United tells us it does not allow the use of the knee defender and the water throwing forced the emergency landing in Chicago on Sunday. Both passengers removed from the plane. The jet continuing. No word on how those two passengers got to Denver. Even though this was an emergency landing, it was considered a customer service issue, not a crime, so neither passenger faces any charges. George and Lara That's kind of surprising. That is surprising after you divert an entire flight. You were saying you had be insane if -- If I was one of the other passengers. Called a knee defender. You would think it's a broken seat. You would think I can't go back.

This transcript has been automatically generated and may not be 100% accurate.

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